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Tuesday, April 28, 2015

The failure

A leading Midwest steel mill plant was
constantly replacing tire couplings on runout tables. Needless to say,
the amount of time required for maintenance was becoming unmanageable.

The equipment and environment

This particular hot strip mill plant
produces a coiled steel rolls commonly used to form large kitchen appliances
and automobiles. Runout tables, containing hundredths of rollers, move
steel slabs along the shaping process. Each roller uses a tire coupling that
attached to a motor, which turns the roll.

High misalignment and a lower service factor between motor and roll can cause high failure rates on the tire
couplings. In addition, each tire coupling has several component
fasteners that require tools to change the element. The design of the tire
coupling makes it susceptible to corrosion from the heat, water, and slag
associated with a runout table application.

The steel mill plant had a dedicated
maintenance team to replace the large quantities of tire couplings every month.
The high replacement part cost, increased maintenance time and resources required to maintain
the tire couplings was prohibitive.

The solution

After a fact-finding visit to the hot strip mill plant,
the Lovejoy Engineering Team analyzed the situation and presented an effective solution.

The team addressed three core runout table
failures:

1) Tire coupling slipping

2) Heat, water, and slag damages

3) Inefficient inspection process.

The recommended solution was to replace all
tire couplings with the Lovejoy’s Jaw In-Shear 6 Pin (JIS) couplings. The
Jaw In-Shear 6 Pin coupling is made up of a standard L-type jaw coupling, a
radially assembled element, and stainless steel locking ring.

With its effective locking mechanism, the
JIS coupling eliminated the slipping issue. The stainless steel locking
ring provided the element with protection from heat, water, and slag. Lastly,
the ease of simple visual inspection and quick change-out, made the JIS the most
effective solution.

In the end, failures were reduced to a minimum. The dedicated plant maintenance team could now refocus its
attention on other critical areas in the mill. Furthermore, this solution
brought in a 33% cost savings per coupling.

Monday, April 20, 2015

The Challenge

A plant manager, from a leading
coal-fired plant located in the Midwestern United States, approached us with a
question about a gear coupling failure in its coal pulverizer drives.

Power
Generation Industry background

More than 50 million consumers
and business owners from Ohio to New York and Canada would agree that a
consistent power supply is not only vital to the overall economy, but also for
the comfort and stability of daily life. The major power outage that struck the
U.S. and Canada in mid-August of 2003 cost New Yorkers alone, an estimated $500
million. Although experts point to a dated power grid as the main cause of the
recent outage, every detail of the power generation system, including the smallest component, plays a critical role in maintaining the electric power plant.

Equipment
and Components

Depending on the size, a
single plant can have more than 25 coal pulverizers. Coal pulverizing allows
for less emissions and more efficient use of the fuel. The coal burns to create
the steam that turns the generators and provides electric power. Large electric
motors use a worm gear arrangement to drive the coal pulverizers and a blower
that sends the pulverized coal to the furnaces. The shafts of the motor must be
mechanically connected to the worm gear and blower. The worm gear is a
relatively small, yet vital component. If it fails, the pulverizers and blowers
can no longer operate.

Dissecting the problem

Gear couplings allow for axial
movement of the shafts, which increases the wear on the gears, sleeve bearings
and other components. This wear on the components would cause them to fail
prematurely.

The loss of production
due to downtime was becoming an issue. Bearing replacement costs were in the
thousands of dollars per bearing.

Preventative maintenance
costs associated with gear couplings were also becoming a burden due to time
spent on inspection and lubrication to keep them operative. The power plant’s
rigorous preventative maintenance program required regular disassembly of the gear
couplings for inspection, cleaning and re-lubrication of the gear teeth. In
addition, assessment of the condition of the couplings was a judgement call
unless extensive and time-consuming measurements were performed. Coupling
replacements also require heavy machinery to move the motors.

Old technology, old solution

The existing gear coupling design was typical of the technology
available when the plant was built in the early 1970’s. Where gear couplings
did not limit axial movement of the motor shaft, sleeve bearings were used.
This old technology configuration increased wear and shortened life spans of
both the motor sleeve bearings and the gear couplings.

In other words, increased
sliding velocity of the gear teeth in the gear coupling creates a lot of heat
and wear, just picture yourself rubbing two sticks together. The motor moves a
bit, the gear on the coupling moves a bit, and this motion could be repeated up
to a million times over the course of the day, contributing to premature
failure.

Contemplating a similar
solution

Install gear couplings
with limited end float; however, its maintenance nightmare will continue
because the gear couplings are difficult to assess for wear and the inspection
and replacement of the gear coupling still required removing the motors. The
gear couplings would still have to be disassembled, inspected, cleaned, and
re-greased every time; definitely not an effective solution.

New
Technology, new solution

Install a Disc Coupling.
Unlike gear couplings, the Lovejoy SX Disc Coupling design is inherently
balanced. There is less vibration than the gear coupling as it wears. The disc
coupling performs a limited end float function without special design
considerations for the sleeve bearings. Maintenance-free operation is achieve
because the Lovejoy coupling has no moving parts and requires no lubrication.
This translates into a long coupling life.

Actual Pulverizer Solution

Preventative maintenance
and inspections of the disc coupling’s flexible elements was now possible using
a simple strobe light and while the coal pulverizer is running. Hence, there is
no need for an expensive shutdown and disassembly. Replacement of the disc
packs became a breeze without moving any machinery or disturbing the original
alignment.

Bonus
features

Lovejoy was able to
provide a heavier duty coupling than the gear type in the same space as before.
Nothing had to be moved or modified for installation. Additionally the
component cost was comparable to those of a gear coupling. The Lovejoy’s design
allows for more capacity and misalignment capability than any other disc coupling
available today.

In addition, disc
coupling signals the maintenance staff when it needs replacement – you can
actually see it with your own eyes without any disassembly. In contrast, in
gear coupling is more of judgment call, unless you take it completely off the
machine and measure it extensively. With
the Lovejoy disc coupling, the outer leaf of the disc will crack before the
entire disc pack fails, signaling to the maintenance staff it needs replacing.

Happy
conclusion

The coal pulverizer is critical equipment, if it is not
blowing coal, it is not generating power; needless to say, would you rather
have your pulverizer blow coal or blow off your money?. The disc coupling
solution reduced the maintenance nightmare, and the plant has been experiencing
success for more than a year with the original installation. Plant management
approved the proposal to convert all gear couplings on 24 coal pulverizers to
Lovejoy disc couplings, based upon the proven benefits and demonstrated cost
savings.

Monday, April 13, 2015

If you've ever been to a major trade show, you will notice that most manufacturers go to great lengths to highlight only their absolute best looking products. With this in mind, we thought we would take a radical approach, and create a booth filled with coupling failures for Hannover Messe 2015 (Hall 25, Stand D34).

That's right, a trade show booth filled with all kinds of product failures. The natural question one might expect is: "Why?" (And sure enough, before the show has even started, one competitor walked up to our booth and said "Why are you showing bad couplings?")

Failed Disc Couplings at Hannover Messe

The answer is simple. We genuinely care about our customer and want to educated them as to what may and can go wrong if they do not size, select, or install a coupling correctly. Having been a market leader in the flexible coupling market for generations, we are also more than confident enough in ourselves and our products to not worry about the impact of highlighting products that were not used correctly.

By design, couplings are generally a wear item with a finite life. Furthermore, they are designed to take the excessive abuse of a system, and are often designed to formally serve as a system fuse.

Primary Hannover Messe 2015 Booth (Hall 20, C36)

If you are lucky enough to be in Hannover, Germany and attending Hannover Messe 2015, we hope you will stop by our Learning Lab to ask whatever coupling question you may have, learn from what we have on display, and are able to grow from the incredibly talented engineers we have staffed the booth with this year.

Will this approach work? While we are quite optimistic, we of course would love your feedback. Please let us know if you think the booth was a good idea, and how we could improve upon it in the future. And, if you really can't stand the idea of seeing worn out couplings... or just would like to be reassured by a whole bunch of those as well... we'll we still have you covered at Hannover Messe. In addition to the Coupling Learning Lab booth, we also have our primary R+L Hydraulics hydraulic & power transmission booth in Hall 20, Stand C36.

Thursday, April 9, 2015

A customer recently called in to Lovejoy concerned about a jaw coupling failure. The unusual aspect of this issue was that three hubs broke in half in one week (see picture 1). Lovejoy’s technical team was baffled by this inquiry and began the process of analyzing the failures.

Picture 1.

This particular application had an electric motor (30 HP) driving
a blower at 3510 RPM. Initially, our engineers focused their attention on over torque. The jaw coupling used in this application had an NBR spider rated for 792 in-lbs. of
torque. The application had a torque rating of 650 in-lbs. Based on the torque specs alone, one could say that the couplings should have performed well under normal conditions (for more details on calculating torque seeCoupling Sizing Torque - How to Quickly Calculate).In conjunction with the over torque troubleshooting, the end user confirmed that the alignment was within the
coupling specifications. The customer used his own methods and tools to verify alignment, and he also followed the Lovejoy Install Videos and Install Guide
step-by-step procedures.

As our technical team continued with the troubleshooting steps, they took the time to share additional technical details as to what can cause torque spikes. One of possible causes is motor pauses and restarts. The customer then mentioned that they had an electrician replace a fuse on the motor because it had blown out previously. Furthermore, the customer mentioned that there were momentary pauses that occurred after replacing the
fuse, which is an indication that the motor was causing torque spikes.

Picture 2.

Although fail-safe by design, jaw couplings can fail catastrophically when hit with a large enough over-torque. (This would be a good time to remind you that OSHA requires coupling guards for all flexible coupling applications. Please be safe.) When a jaw coupling hub fails catastrophically, it will typically fail either by shearing at the jaw legs, or by breaking open between the inside bore and outside diameter of the hub (generally along the keyway - see picture 2).In this particular case and because of torque rating specs, it turned out the coupling breaking in half was actually a good thing. If the couplings were to continue to transfer torque between the motor and the blower, the probability of burning the motor or damaging the bearings in the blower becomes high. Breaking up (coupling failure) allows you to spend a few dollars to fix the problem as opposed to thousands of dollars in equipment replacement.